The invention relates to a method of producing granular sodium percarbonate by means of fluidized bed granulation.
Various methods are known for producing sodium percarbonate with the formula 2 Na.sub.2 CO.sub.3.3 H.sub.2 O.sub.2. One method involves the conversion of hydrogen peroxide with sodium carbonate in aqueous phase, crystallization of the sodium percarbonate and the separation of the sodium percarbonate from the mother liquor. A second method requires conversion of solid soda with aqueous hydrogen peroxide. A third method includes fluid bed granulation, wherein a hydrogen peroxide solution and a soda solution are sprayed into a fluidized bed apparatus onto nuclei of sodium percarbonate and at the same time water is evaporated.
The first method described above is used on an industrial scale; however, auxiliary substances such as sodium chloride for salting out and metaphosphates for controlling the crystallization are required. In addition, a purification and/or partial discharge of the mother liquor is/are necessary in order to achieve a good product quality.
The quality of sodium percarbonate produced according to the second method described above is usually not comparable to that of sodium percarbonate produced according to either the first or third methods on account of inhomogeneities which occur and on account of unsatisfactory storage stability.
The use of the third method described above is becoming increasingly more interesting because it does not result in any accumulation of waste water. It also results in a very high yield in wear-resistant sodium percarbonate. German Patent 20 60 971 teaches such a method. This patent document is entirely incorporated herein by reference. According to this method, a fluidized bed containing sodium percarbonate nuclei whose dimensions are smaller than those of the granulate particles to be produced is continuously charged with a sodium percarbonate solution or sodium percarbonate suspension. Alternatively, the bed is separately and simultaneously charged with an aqueous hydrogen peroxide solution and an aqueous sodium carbonate solution, and water is continuously evaporated from the aqueous environment containing sodium percarbonate. Granulated particles of a certain size are drawn out of the fluidized bed. When a sodium percarbonate solution or an H.sub.2 0.sub.2 solution and an Na.sub.2 CO.sub.3 solution is/are used, nuclei are introduced simultaneously into the fluidized bed.
The method described above exhibits a number of disadvantages. In one embodiment, a sodium percarbonate solution or suspension must first be produced, which necessitates an additional method step. The charging of a fluidized bed with a sodium percarbonate suspension or solution supersaturated with sodium percarbonate is, in addition,,prone to problems because the injection nozzles rapidly becomes clogged. On the other hand, if a dilute sodium percarbonate solution is used, much water must be evaporated. This increases the expense.
German Patent 27 33 935 points out problems of a further embodiment taught in German Patent 20 60 971 C3. German Patent 27 33 935 also is entirely incorporated herein by reference. As described in German Patent No. 27 33 935, when using an aqueous hydrogen peroxide solution and an aqueous sodium carbonate solution employing two separate injection nozzles, e.g., customary two-component nozzles for spraying a solution with co-usage of air as a propellant, it is difficult to achieve a sufficiently intimate mixture of the two solutions in the fluidized bed. However, it is necessary to have such intimate mixing in order to obtain homogeneous sodium percarbonate particles. If the two solutions are introduced in common into the fluidized bed through a single injection nozzle, a crystallization usually takes place in the injection nozzle after a brief operating time, which results in clogging and interruptions of the operation.
In order to eliminate the above-mentioned problems, German Patent 27 33 935 C2 suggests using a common injection nozzle for both solutions and, in order to avoid a clogging of the injection nozzle, dissolving a metaphosphate in at least one of the two solutions. The mixing of the two solutions takes place in the interior or at the inlet of the injection nozzle. The amount of metaphosphate used is advantageously between 0.1 and 20 grams per kg sodium percarbonate.
The co-usage of a metaphosphate in the method of German Patent 27 33 935 C2 is undesirable because it elevates the expense for raw materials. Additionally, the phosphate component is introduced into the sodium percarbonate, and therewith into the washing, bleaching and cleaning agents containing the sodium percarbonate. Those skilled in the art are becoming increasingly more interested in eliminating phosphates from such products for ecological reasons.